Numerical study on the fire behaviour of restrained steel-concrete composite beams

An elaborate solid-shell finite element model was developed to investigate the fire behavior of restrained steel-concrete composite beams. The model was validated against experimental results in literatures. Parametric studies were also performed to study the parameters including restraint stiffness, load ratio, span, exposure length, embedding depth, steel beam height and protective layer on the fire behavior of restrained composite beam. The analysis results show that the mechanical response of restrained composite beams goes through four phases: elastic, elasto-plastic, rapid plastic deformation and plastic catenary action phase. With the decrease of restraint stiffness, the increase of load and span, the failure mode of the restrained composite beam changes from overall lateral instability to a sharp increase in deformation rate, and the code BS476-20 and ISO834 underestimate the fire resistance of restrained composite beams. The negative bending moment at the beam end increases sharply and reaches the peak value within a short fire time. Therefore, reinforcement shall be provided at the top of the concrete slab near the support section. Furthermore, the difference in fire resistance of restrained composite beams is mainly reflected in the plastic catenary action phase. When the load ratio is less than 0.5, the fire resistance of restrained composite beams can be improved by reducing the fire range, beam height and span, and increasing the axial restraint stiffness. When the load ratio is 0.7, the fire resistance of restrained composite beam is hardly affected by the axial restraint stiffness and span, while with the exposure length reducing from 5 m to 4 m, embedded depth increasing from 0 mm to 50 mm, and the protective layer increasing from 0 mm to 10 mm, the fire resistance of the composite beams increases by 114 min and 160 min and 76 min, respectively.